It is often necessary to determine the characteristics of an unknown alternating current. This can be done by measuring the magnitude of the current while independently determining its frequency. Measuring the magnitude of an unknown alternating current typically involves connecting the unknown alternating current through a known precision calibrated resistance device and measuring the voltage across the resistance element, or comparing a known direct current with that of the unknown alternating current by measuring the difference through a precision thermal current-to-voltage converter. Connecting the unknown alternating current through a precision resistance device is usually the preferred alternate technique as it is a less complicated procedure and because of the availability of precision alternating voltage meters.
In conventional measurement techniques, the measurement of the magnitude of an unknown alternating current is accomplished by connecting the unknown current through the calibrated resistance, commonly referred to as an alternating current shunt. A voltage reading across the resistance is taken and a simple Ohm's law calculation can be made to determine the current magnitude. If the frequency is not known, a frequency counter can also be connected across the resistance to determine the frequency of the alternating current. One skilled in the art will appreciate that though the discussion thus far has centered around voltage magnitude and frequency determinations, the phase relationship between the alternating current and the resulting voltage across the resistance cannot solely be determined using this technique. The voltage to current phase relationship depends on the impedance characteristics of the calibrated resistance device. These impedance characteristics will introduce increasingly significant errors in the measurements at higher frequencies. Both the effects of capacitance and inductance components, parasitic or otherwise, of the resistance device must be considered.
To provide high precision measurements of alternating current, the voltmeter must be suitably accurate and the voltage to current phase relationship of the calibrated resistance device must be known. Typically, voltmeters are most inaccurate when measuring low voltages, as typically the error in a voltmeter measurement is absolute and not relative. For low value resistances or low value currents, even a very sensitive voltmeter pushes towards an inaccurate region unless the resistance value and current to be measured are suitably matched. For higher value resistances the effect of even a small capacitive component of the voltmeter input terminals and cable connections will significantly reduce the accuracy of the voltmeter readings at the higher frequencies. On the other hand for low value resistances the inductive component of the connections will significantly increase the voltmeter readings at the higher frequencies, thus diminishing the accuracy of the result.
The process of selecting suitable resistance values for a particular shunt must take into account the power that will be dissipated during the measurement process. The measurement of currents of increasing magnitude requires greater power dissipation in the calibrated resistance device if the resulting voltage magnitude requirement is to be maintained. As either current or voltage increase linearly, the power dissipated by a fixed resistance increases as a square (P=I2R=V2/R). Typically a voltage above 200 millivolts is required to maintain measurement accuracy levels approaching 0.001%. For instance, an alternating current shunt in the range of 50 A to 100 A with a nominal resistance value of 0.004 ohms provides a voltage value in the range of 200 millivolts to 400 millivolts and would be required to dissipate 40 watts at the high end of this range. To provide sufficient thermal dissipation and prevent damaging the shunt, multiple resistive elements are typically used in the calibrated resistive device to provide sufficient surface area from which to dissipate the heat generated by passing the current through the resistive elements. In order to dissipate this thermal energy with a reasonable rise in temperature, a thermal path of sufficient area and thermal conductivity must be provided between the resistance elements and the surrounding environment.
Conventional designs increase the dissipative ability of the resistive device by making use of a number of resistive elements. The individual resistances are carefully arranged to reduce the adverse effects on the measurement.
One conventional design is a coaxial design making use of relatively long resistive wire elements. This design provides for a relatively flat frequency response but has poor temperature and power coefficients. The poor temperature and power coefficients are due primarily to limitations of the available resistance wire with temperature coefficients lower than 10 ppm/C. The overall result is poor thermal stability and very long stabilization periods (which can be on the order of a few hours) at currents above 1 ampere.
Another conventional design is a basic radial design utilizing metal film resistive elements with relatively high temperature coefficients similar to that of the coaxial design. In order to improve the thermal stability, the assembly utilizes an open enclosure design to reduce both the heat effects and the stabilization time. This results in an unshielded shunt which makes the element subject to a number of undesirable effects. A variation on this design makes use of a shielded enclosure with encapsulated resistive elements. The encapsulent interferes with the thermal dissipation of the generated heat and results in very long stabilization times at higher currents.
The final conventional design presented here is known in the art as a rectangular box design. This design is primarily designed to optimize the frequency response to 100 kHz and above. The size of the box required for currents above 10 amperes is considered excessive and the cost is very high due to the complicated method of fabrication. This design does not provide an electrically shielded enclosure and thus is susceptible to failure if it encounters metallic objects in the immediate vicinity. This design has been limited in its use and deployment due to the high cost, environmental, and safety limitations.
The phase relationship of the voltage to the current is dependant on the physical arrangement of the resistive components in the AC shunt. A number of techniques have been used to minimize the inductive and capacitive components that affect the impedance characteristics of the resistive device. The residual inductive and capacitive components result in a frequency dependency in the effective resistance of the device. The effect is such that measurement errors are introduced unless this dependency is known a priori and taken into account in the measurement process. For resistance values below approximately 10 ohms a residual inductive component that can be modeled as being in series with the current path predominates as the frequency dependent component of the nominal resistance. Above 10 ohms a residual capacitance component that can be modeled as being in parallel with the current path predominates as the frequency dependent component. An inductive component of 50 nanohenries will increase the shunt resistance the equivalent of 31 milliohms in series at 100 kilohertz and a capacitance component of 10 picofarads will decrease the resistance the equivalent of 160 kilo-ohms in parallel with the shunt at a frequency of 100 kilohertz.
The design of precision alternating current shunts must take into consideration the minimization of inductive and capacitive components as well as the minimization of the effects of power dissipation.
Therefore it is desirable to provide an alternating current shunt design that minimizes changes in the effective resistance value with increasing current as well as with increasing frequency over a specific measurement range.